between them. It has been a prominent part of APC ever since
computers acquired the necessary computational capabilities to perform matrix-math based control and optimization
algorithms in the 1980s. This computing ability allows MPC
to control multiple variables simultaneously with complete
understanding of their complex relationships in real time.
Because of the number of independent and dependent variables and the complexity of the relationships concerning dome
temperature, the team recommended a design solution using
an MPC controller. The controller will use manipulated variables such as steam flow to influence the controlled variable
of dome temperature. It will also consider controlled variables
such as minimum discharge temperature, DT motor amps, and
maximum dome pressure to keep its steam flow manipulations
within necessary constraints. Using disturbance variables that
influence the controlled variables, the MPC controller knows
how much the discharge temp will drop before it changes steam
flow, thereby altering the steam flow without under correcting
or over correcting.
For this situation, we want the MPC to use the following
• Control dome temperature to a target value.
• Maintain discharge temperature above a limit.
• Maintain dome pressure within limits.
• Maintain gearbox amperage within set limits.
• Maintain deck pressure within set limits.
• DT top deck level
• extractor speed
• Steam control valve
All of these goals and restrictions were written in the MPC
programming at the time of implementation. Cost for the hard-
ware (MPC controller, SCADA servers, operator HMIs, PLC
controllers, and Ethernet capability), software programming,
installation, and on-going support were calculated during ROI,
and installation and implementation went smoothly.
The recommended MPC control began testing and moni-
toring in August 2014. Our data collection from August through
October show a marked decrease in steam consumption and
50% decrease in dome temperature variance compared to the
data before APC installation. Initial ROI numbers forecast that
savings for the plant would surpass cost in 19 months. The real-
time data from daily operations indicate the break-even point
to be less than 12 months. Savings on the project is equal to
approximately 1 pound of steam per bushel of soybeans pro-
cessed. Since the implementation of the project, the plant has
experienced no unplanned shutdowns due to the dome temper-
ature dropping below its safe threshold. Necessary constraints
have been honored with no instances of operation that violate
any of the MPC disturbance variables, just as we hoped. The
plant is on track to save more than $150,000 annually as a result
of installing one APC to control one variable.
For continuous industrial processes already in production, improving operational efficiency or increasing productivity can come from implementing some form of APC on top of
basic processes. This project was a small step to see how APC
might help control a widely-variant DT dome temperature.
Beyond controlling steam flow for this single process, the plant
now has a path forward to greater profit with small risk. This
same process flow has close to a dozen other points that may
benefit from further APC control, leading to even greater cost
savings within the plant. Other plants within the company could
also improve their profit margins by implementing the APC
measures used successfully at this facility. The company could
improve profit margin greatly over the long-term without hiring
personnel, without increasing consumption, without purchasing more raw materials, and without changing product pricing.
APC provides a logistically sound way to manage resources
better within specified boundaries. This specific APC implementation is a good example of how seemingly small adjustments can provide substantial results over time.
Jim Vortherms is a division manager for Interstates Control
Systems, Inc., Sioux Center, Iowa, USA. He can be contacted